1. Field of the Invention
The present invention relates to a method of producing pigment dispersions suitable for manufacturing characteristic color pigment dispersions and black pigment dispersions useful as an inkjet recording material.
2. Description of the Related Art
In recent years, digital printing technologies are powerfully progressing. These digital printing technologies are represented by technologies referred to as electrophotography technology and inkjet technology, and increasingly strengthen their presence as image forming technologies in offices and homes in recent years.
Among these, the inkjet technology has advantageous features of compactness and low power consumption as a direct recording method. Further, image quality is rapidly being enhanced by making the nozzle fine, etc. An example of the inkjet technology is a method in which ink fed from an ink tank is bubbled by heating the ink by a heater in a nozzle and images are formed on a recording medium by ejecting the ink. Another method involves ejecting ink from a nozzle by vibrating a piezo device. Such an inkjet system can be used for various applications using coloring materials for recording. Specifically, it can be used in stationary, color printers, color plotters, poster printing, signboard printing, quick printing, textile printing, color filters for liquid crystal displays and the like.
Although water-soluble dye inks have been applied in these methods, phenomena referred to as bleeding, feathering, and weathering sometimes occur. Use of a pigment dispersed ink for improving these problems is disclosed in U.S. Pat. No. 5,085,698.
However, pigment inks are often inferior to dye inks in the long-period preservation stability and the stability in ejecting from an inkjet head. Further, since light scattering and light reflection due to pigment particles occur, images formed of pigment inks generally have a tendency of having a lower color developability than those formed of dye inks. Moreover, coarse pigment particles cause nozzle clogging of an inkjet head. Attempts to make pigment particles fine as a method of solving these problems of pigment inks are being made. Pigments made fine into not more than 100 nm are less affected by light scattering and have a larger specific surface area, so improvement in color developability is expected. A method of making pigments fine generally involves milling such as sand milling, roll milling or ball milling. This method takes much energy and time to prepare a pigment dispersion having a particle size of not more than 100 nm and a narrow particle size distribution, so it is not an easy method.
As means for further improving image quality and color reproducibility, color printers using characteristic color inks such as orange, green and blue ones as well as conventionally used inks such as yellow, magenta, cyan and black ones have being commercially available in the fields of large format, texture and other printings. While a single pigment exhibiting a color development other than yellow, magenta and cyan are used as characteristic color inks, a method of producing characteristic color inks through color mixing is also disclosed. For example, a method is disclosed in which a yellowish red pigment is manufactured by milling together a red organic pigment and a yellow organic pigment (Japanese Patent Application Laid-Open No. 2004-091602). A method is disclosed in which a deep blue pigment is manufactured by milling together copper phthalocyanine and dioxane violet or by the acid paste method in which a solution obtained by dissolving them together in a concentrated sulfuric acid is poured in water to reprecipitate the pigment (Japanese Patent Application Laid-Open No. 2002-003739).
Carbon black is most widely used as a black pigment, and is known to have a high coloring power. On the other hand, Japanese Patent Application Laid-Open No. 2003-041144 points out that carbon black contains carcinogenic polycyclic condensation compounds, and discloses a manufacturing method of a black perylene-based pigment as an alternative to carbon black. According to this method, the black perylene-based pigment is manufactured by high-temperature calcination under an inert gas atmosphere; therefore, control of its particle size and particle size distribution is not easy, and much improvement is still desired for obtaining a dispersion having a uniform particle size.